1. Field of the Invention
This invention relates to control circuits for use in optical transmitters for analog RF signals, and in particular to externally modulated solid state lasers. More particularly, the invention relates to the use of a single integrated circuit coupled to the external modulator of the laser for continuously adjusting the bias of the two RF inputs to the modulator in order to minimize composite second order beat (CSO) distortion.
2. Description of the Related Art
Modulating the analog intensity of the optical signal from a light-emitting diode (LED) or semiconductor laser with an electrical signal is known in the art for transmitting analog signals such as sound and video signals, on optical fibers. Although such analog techniques have the advantage of significantly smaller bandwidth requirements than digital pulse code modulation, or analog or pulse frequency modulation, amplitude modulation puts stringent requirements on the noise and nonlinearity associated with the optical source.
For that reason, direct modulation of the laser has been used in connection with 1310 nm lasers where the application is to short transmission links that employ fiber optic links with zero dispersion. For applications in metro and long haul fiber transmission links the low loss of the link requires that externally modulated 1550 nm lasers be used, but such external modulation techniques are more complex mixture of the number and type RF channels, with modulation types ranging from analog to QAM. The present invention is therefore addressed to the problem of providing a simplified hardware arrangement using a single integrated circuit processor which, under control of a software algorithm, simultaneously adjusts the bias of the two RF inputs of the external modulator so that the optical output signal can be used in single mode fiber used in metro and long haul optical networks.